YouTube – How to Prevent Autoplay

I know you asked for Firefox explicitly, but if you were to move to Chrome you could use the Stop Autoplay for YouTube extension (which stops playback yet allows buffering) combined with the Auto HD for YouTube to automagically select your preferred quality levels.

Both plugins can be found on Google's Chrome Extensions.

EDIT:
For FireFox, you could install GreaseMonkey and use this script which does what you want (and supposedly blocks adverts as well): http://userscripts.org/scripts/show/49366

Right-click on the video and select Stop downloading. To start it up again click the pause/play button.

Another way is to move the slider all the way to the end or near end of the video timeline. That way only the few remaining seconds left to play will end up streaming down to your computer and when it finishes, that's it. No more buffering or loading.